1. Field of the Invention
The present invention relates to a method of manufacturing a radial tire, and more specifically to a method of manufacturing a so-called low section radial tire available for high performance passenger cars and formed with an improved auxiliary layer arranged on the outermost belt layer.
2. Description of the Prior Art
In low section radial tires for high performance passenger cars, a plurality of belt layers are formed at the crown portion of a carcass in such a way that cords of each belt layer are inclined at a small angle with respect to the tire equatorial plane and cords of different belt layers are laid one upon another so as to intersect each other. Further an auxiliary layer made of heat shrinkable cord is wound around the outermost belt layer along the tire circumferential direction, viz. in parallel to the tire equatorial plane, in order to improve the hoop-effect of the belt layers.
In forming the above-mentioned low section radial tire, two belt layers 50 are wound around a flat drum D (shown in FIG. 1); an auxiliary layer 60 is wound around the outermost belt layer 50 so as to cover the entire width of the belt layer; and a tread rubber 30 is bonded onto the auxiliary layer to form a belt structure body B. On the other hand, a carcass structure body is formed in a carcass forming process different from the above belt forming process. The formed carcass structure body 40 is expanded into a toroidal shape (not shown); and the already-formed belt structure body B is fitted to the outer peripheral surface of the carcass structure body 40 so as to uniformly cover the carcass, in order to form a green tire (not shown). Thereafter, the formed green tire is vulcanized or hardened within a mold (not shown) into a finished tire.
In the prior-art method of manufacturing the radial tire, there exists a problem such that the coiling diameter of cords 61 of the auxiliary layer 60 of the finished tire become smaller at both the side areas W.sub.2 (shown in FIG. 2) of the tread 30 than at the central area W.sub.1 of the tread 30, so that the initial cord tension or initial cord elastic modulus is smaller at the side areas W.sub.2 than at the central area W.sub.1.
This problem gives rise to a decrease in hoop effect at the side areas W.sub.2 as compared with that at the central area W.sub.1, so that the two belt layers 50 deform radially outward due to centrifugal force generated when the tire is rotating at high speeds. Therefore the belt layers are excessively fatigued, resulting in belt end separation trouble, poor high-speed tire durability, and tire non-uniformity.
Originally, the auxiliary layer 60 is additionally arranged to reduce tire radially-outward deformation at high speed. However, since the initial cord tension is smaller at both the sides of the tread, the arrangement of the auxiliary layer is not effective.